Colossians 2: Reality in Christ

Jesus is the King who conquered to us a new life so that we became citizens of the kingdom of God. The Colossian church was under attack. Bad teachers were teaching how to be more "holy" through circumcision, keep the Sabbath, kosher food, etc.
We read:

16-17 So don’t put up with anyone pressuring you in details of diet, worship services, or holy days. All those things are mere shadows cast before what was to come; the substance is Christ.

18-19 Don’t tolerate people who try to run your life, ordering you to bow and scrape, insisting that you join their obsession with angels and that you seek out visions. They’re a lot of hot air, that’s all they are. They’re completely out of touch with the source of life, Christ, who puts us together in one piece, whose very breath and blood flow through us. He is the Head and we are the body. We can grow up healthy in God only as he nourishes us.

Paul warned: We are holy through Jesus because he is the substance, while diet, Sabbath, etc. are only shadows of Jesus. There is a strong trend in our human hearts to find some holiness in our own effort, but this is a lie. We are made to look to Jesus.